Carbon nanotubes are hollow cylinders of carbon atoms. Their appearance is that of rolled tubes of graphite, such that their walls are hexagonal carbon rings, and they are often formed in large bundles. The ends of carbon nanotubes are domed structures of six-membered rings, capped by a five-membered ring.
With the possession of metallic conductivity and semiconductor conductivity according to structures, carbon nanotubes are now top candidate to be applied to various technological fields such as, for example, electrodes of electrochemical storage devices (e.g., secondary cells or supercapacitors or fuel cell), electromagnetic shielding, field emission displays or gas sensors.
Generally, the production amount of carbon nanotubes (CNTs) is small because hands are still in charge of performing many their production steps including a step of loading/unloading a CNT-compounded substrate on/from a reaction tube and a step of unloading the substrate from the reaction tube to retrieve a CNT therefrom. Hence, it is difficult to perform successive process and mass production of carbon nanotubes.
Larger diameters of reaction tubes are necessary for mass production of carbon nanotubes. For this reason, multi-stage/multi-column boats have been required. However, a multi-stage/multi-column boat has a great deviation in gas density (gas density at front and rear columns of the boat and gas density at upper and lower stages of the boat) according to the positions of composite substrates. Generally, source gas flows down to the bottom of a reaction tube because it is heavy. Thus during a process, the source gas is excessively concentrated on a composite substrate disposed on a front-column lower stage while a relatively small amount of source gas is supplied to a composite substrate disposed on a rear-column upper stage. As a result, the entire productivity of carbon nanotubes decreases.
Since hydrogen-containing source gases (noxious/explosive gases) are mainly used to compound carbon nanotubes, it is necessary to eliminate residual gases inside a reaction tube. Unless source gases inside the reaction tube are fully exhausted after compounding carbon nanotubes, a noxious gas (hydrogen) among residual gas elements inside the reaction tube may be leaked to the air and react to oxygen to be exploded. Especially, a possibility of accident resulting from residual gases may continue to increase with the recent trend toward larger diameters of reaction tubes.
As diameters of reaction tubes become larger and the number of composite substrates required in a process is increasing, gas density deviation based on the positions of composite substrates (substrates disposed at a front column and a rear column) becomes higher. The gas density deviation leads to deterioration in efficiency of a process for compounding carbon nanotubes that are sensitive to a gas uniformity. Further, carbon nanotubes fall on the bottom inside a reaction tube while retrieving composite substrates compounded from the reaction tube. The carbon nanotubes remaining at the reaction tube result in contamination of the interior of the reaction tube and malfunction of a robot provided for transferring composite substrates and have an adverse effect on the flow of source gases.